1. Field of the invention
This invention relates generally to an ultrasonic probe using a piezoelectric crystal material in an ultrasonic diagnostic apparatus and more particularly to an ultrasonic probe having metal layers between the piezoelectric crystal and the backing.
2. Discussion of the Background
Ultrasonic imaging apparatuses using an ultrasonic probe are involved with medical diagnostic apparatuses used to inspect body interiors, defect detecting apparatuses used to examine defects within metallic structures, and so on. In medical diagnostic apparatus, a color flow mapping (CFM) method has been developed that is adapted to display blood-flow speeds as two-dimensional color images by using ultrasonic Doppler shift due to blood flow, in addition to images of human bodies (B mode images). The center frequency of an ultrasonic probe is designed to be different depending upon an object to be diagnosed. Meanwhile, the center frequency for obtaining B mode images differs from the frequency of Doppler modes (Doppler reference frequency) for obtaining CFM images in that, for example, the former is at 3.75 MHz and the latter at 2.5 MHz.
The Doppler reference frequency is set lower than the center frequency in order to reduce the effect of attenuation in a human body. That is, the Doppler mode utilizes reflection echoes due to microscopic blood cells, and accordingly the available signal level is low.
Consequently, the ultrasonic probe should have a high signal level at a Doppler reference frequency, in addition to a B-mode frequency. In other words, there is a demand toward enhancing probe sensitivity over a wide range of frequencies.
In order to enhance the ultrasonic probe sensitivity, a low acoustic impedance material may be employed as a backing material, to reduce residual vibration, at a backside of an ultrasonic transmitting/receiving device.
For the ultrasonic probes in general production utilizing ceramics, the enhancement in sensitivity can be certainly achieved at frequencies around the center frequency of a probe by the above method. On the other hand, however, there is a reduction in sensitivity at frequencies distant from the center frequency, resulting in a narrowed bandwidth.
Another major problem lies in that there is degradation in workability in the dicing process where a low acoustic impedance material is employed as a backing material.
The present ultrasonic probe, having many strip-formed oscillators arranged in an array form, is manufactured by first adhering a large-sized oscillator plate to a backing material and then evenly cutting or dicing the oscillator at a fine pitch by using a dicing saw or the like, thus providing a group of oscillators arranged in an array. The pitch of dicing is as fine as several 10 .mu.m to several hundreds .mu.m. With such a pitch, the degraded cuttability to the oscillator results in a factor of reducing the probe manufacture yield. There is a tendency for the low acoustic impedance material as stated above to be softer than a material with high acoustic impedance. Due to this, certain disadvantages are encountered during dicing.
First, if a backing material is soft, the oscillator is not firmly fixed during dicing. During cutting, the oscillator is unstable and chatters due to the stress applied by the cutting blade. Further, the dicing saw blade tends to be loaded with the soft backing material, resulting in deterioration in cuttability. These factors, in turn, cause small chipping or cracking on each strip-formed oscillator, leading to disconnection of electrode layers of the oscillator, sensitivity reduction, etc. As a result, probe manufacture yield is not satisfactory.